Previous work in this laboratory has shown that significant changes in the apparent fluidity of the plasma membrane of chick heart cells occurs during embryonic development. Developmental changes in the lipid composition of the plasma membrane are partly responsible for the changes in fluidity. We have also shown that marked changes in the function of various membrane transport processes occur during embryogenesis. One purpose of the proposed research is to determine whether the changes in fluidity are casually related to the developmental changes in transport activities. The main question we are asking is: what are the influences of altered membrane fluidity on the function of membrane transport processes? We will begin with studies on red blood cells because the plasma membrane of the red cell is well-characterized biochemically and with respect to its transport processes. We will alter the fluidity of the red cell membrane by anesthetic alcohols and by depleting the membrane of cholesterol, exchanging some of its cholesterol for a noncondensing steroid, enriching the membrane with cholesterol, or by changing the fatty acid composition of membrane phosphatidylcholines by means of an exchange protein. The apparent fluidity of the membranes of the treated red cells will be estimated both by fluorescence and spin-label membrane probes. The transport processes that will be characterized in the altered cells will include: simple diffusion (erythritol, thiourea, lactamide, and others), facilitated transport (glycerol, galactose, uridine), active transport (ouabain-inhibited Na ion and K ion fluxes), and water transport. In chick embryo heart our strategy will be to fluidize the plasma membranes of 5-day hearts to an extent that they resemble 10-day hearts in fluidity. We will determine whether various transport processes in the fluidized 5-day heart resemble those in 10-day hearts. Transport processes to be studied include: simple diffusion, facilitated transport (glucose, sorbitol), water transport and active transport (amino acids).